1. Field of the Invention
The invention relates to an exhaust gas control system for an internal combustion engine.
2. Description of the Related Art
In order to reduce the amount of nitrogen oxides (hereinafter, referred to as “NOx”) contained in the exhaust gas discharged from an internal combustion engine, an exhaust gas recirculation system that returns part of the exhaust gas to an intake system is frequently used (hereinafter, returning part of the exhaust gas to an intake system of an internal combustion engine will be referred to as “EGR”).
In order to make it possible to perform EGR in a broader operating range of an internal combustion engine, the following technology is frequently employed. According to the technology, a high-pressure EGR device that returns the exhaust gas upstream of a turbine of a turbocharger to a portion of an intake passage, which is downstream of a compressor, and a low-pressure EGR device that returns the exhaust gas downstream of the turbine to a portion of the intake passage, which is upstream of the compressor are both provided. Depending on the operating state of an internal combustion engine, the EGR device used is switched between the high-pressure EGR device and the low-pressure EGR device or the high-pressure EGR device and the low-pressure FOR device are used in combination.
Japanese Patent Application Publication No. 2005-76456 (JP-A-2005-76456) describes an EGR system that incorporates the technology described above. According to this publication, when the load placed on an internal combustion engine is low, EGR is performed by a high-pressure EGR device. When the load placed on the internal combustion is high, EGR is performed by the low-pressure EGR. At or around the boundary between the operating range where the engine load is high and the operating range where the engine load is low, the low-pressure EGR device and the high-pressure EGR device are used in combination.
In order to increase the efficiency of reduction in the amount of NOx discharged (hereinafter, referred to as the “NOx discharge amount”), the amount of exhaust gas returned to an intake system of an internal combustion engine by the EGR device (hereinafter, referred to as the “EGR gas amount”) needs to be increased. Namely, in order to increase the efficiency of reduction in the NOx discharge amount, EGR needs to be performed at a high EGR rate.
However, if the EGR rate increases, an increase in the rate, at which the internal combustion engine consumes the fuel (hereinafter, referred to as the “fuel consumption rate of the internal combustion engine”), due to performance of EGR is no longer ignored.
For example, when the amount of exhaust gas recirculated by the high-pressure EGR device (hereinafter, referred to as the “high-pressure EGR gas amount”) increases, the temperature of the gas flowing into a combustion chamber of the internal combustion engine (hereinafter, referred to as the “inflow gas temperature”) increases, which may increase the fuel consumption rate. Also, the fuel consumption rate may be increased by various factors such as a decrease in the air-fuel ratio, an increase in the combustion temperature, and an increase in the amount of smoke (unburned fuel).
If the high-pressure EGR gas amount increases, the amount of exhaust gas that drives a turbine of a turbocharger decreases, which decreases the boost pressure. In contrast, in an internal combustion engine that is provided with a variable nozzle turbocharger, a decrease in the boost pressure is suppressed by reducing the opening amount of a variable nozzle. However, if the opening amount of the variable nozzle is reduced, the difference in pressure between the exhaust gas upstream of a turbine and the exhaust gas downstream of the turbine increases. As a result, the fuel consumption rate may increase due to the pumping loss that is caused by such increase in the pressure difference.
Meanwhile, when the amount of exhaust gas recirculated by the low-pressure EGR device (hereinafter, referred to as the “low-pressure EGR gas amount”) increases, the opening amount of an exhaust throttle valve or an intake throttle valve may be reduced to increase the pressure difference between the exhaust gas downstream of the turbine and the intake gas upstream of the compressor. In this case, the pressure difference between the gas upstream of each of the exhaust throttle valve and the intake throttle valve and the gas downstream thereof (hereinafter, referred to as the “upstream-downstream pressure difference”) increases. As a result, the fuel consumption rate may increase due to the pumping loss caused by such increase in the pressure difference.
An EGR gas passage of the low-pressure EGR device tends to be longer than an EGR gas passage of the high-pressure EGR device. Accordingly, if the low-pressure EGR gas amount increases, the fuel consumption rate may increase due to the passage resistance of the EGR gas passage.
As described above, there are various factors that contribute to increases in the fuel consumption rate of the internal combustion engine (the pumping loss, the inflow gas temperature, the opening amount of the variable nozzle, etc. Hereafter; these factors will be collectively referred to as the “fuel consumption rate increase factors”). The fuel consumption rate increase factors have different correlations with changes in the high-pressure EGR gas amount and the low-pressure EGR gas amount. Accordingly, when EGR is performed at a high EGR rate, it is important to control the ratio at which the high-pressure EGR gas and the low-pressure EGR gas are mixed together (hereinafter, referred to as the “mixture ratio between the high-pressure EGR gas and the low-pressure EGR gas”) based on the correlation between the mixture ratio and the fuel consumption rate in order to suppress an increase in the fuel consumption rate.